Contrary to to the BH case, the echoes found in the wormhole background do not decay with time, but have constant and equal amplitude to that of the initial ringdown. The constancy of the amplitude of echoes is related to the absence of dissipation and may be an indication of the existence of normal oscillation modes, as well as potential instabilities, similar to that found in [1]}. The introduction of gravitational perturbations (such as the ones considered in [2]}, [3]}, [4]}, [5]}) may lead to an unstable wormhole, due to the presence of phantom matter, which can potentially expand or collapse into a BH [6]}, [7]}.
